In said patent application, the deleterious gas stream which could arise via the sealed lubricating oil collection vessel between the chamber of the bearing on the compressor delivery side, which is at the highest pressure, and the chamber of the bearing on the compressor suction side, which is at the lowest pressure, and could entrain oil vapour and droplets through the labyrinth seal and towards the impeller of the first compressor stage with a consequently unacceptable loss of lubricating oil, is compensated by including in the discharge pipe to the vessel from said chamber on the compressor suction side a dynamic barrier of liquid ring type which allows the lubricating oil to pass by gravity but prevents passage of any gas or vapour in the opposite direction.
More specifically, said dynamic barrier consists of a disc rotating within a toroidal stator part which with the disc forms a small passage which remains always filled with barrier oil by the centrifugal effect of the rotating disc.
Although this solution is highly effective as a barrier, it suffers from a series of drawbacks such as a certain constructional complexity and cost due to the need for an auxiliary motor for driving the disc, and a high dissipated power because the entire quantity of oil necessary for lubricating the bearing on the compressor suction side has to pass through the passage between the rotating disc and the toroidal stator part.